1. Field of the Invention
The present invention relates to a mask for masking a non-wiring area on a board or substrate in order to form a fine metal wiring on the board, for example, by means of the vapor deposition, and a method for producing the mask. The present invention also relates to a method for producing a wired board by forming a wiring on a board by using the mask.
2. Description of the Related Art
In order to form metal wirings on a board or substrate, a film of metal is formed and processed on the surface of the board. The method for processing the metal includes, for example, the subtractive method, the additive method, and the lift-off method. In the case of the subtractive method, a metal film is previously formed on the entire surface of the board, for example, by means of the sputtering method. A resist pattern is formed on portions to be converted into wirings, and unnecessary portions are removed, for example, by means of the etching. In this method, the accuracy of the formed wiring is affected by the accuracy of the etching. Therefore, this method involves such a drawback that it is difficult to apply this method to the formation of fine wirings. In the case of the additive method, a resist pattern is formed on portions of the board surface on which any wiring is unnecessary. A metal film is formed on only portions at which the resist is absent. In this method, the accuracy of the formed wiring is determined by the accuracy of the resist pattern. Therefore, it is possible to form fine wirings. However, this method involves such a problem that when a plating film or the like is formed, then the heat tolerance of the film is weak, and the reliability is low.
In the case of the lift-off method, a resist pattern is formed at portions at which no wiring is required. A metal film is formed on the entire surfaces of the board and the resist, for example, by means of the sputtering method. After the film formation, the resist and the metal film on the resist surface are exfoliated to form the wiring. In this method, it is possible to form the fine wiring. Additionally, it is possible to form the metal film having the high heat tolerance by means of the sputtering method with ease. It is possible to solve both of the problem concerning the accuracy involved in the subtractive method and the problem concerning the heat tolerance of the film involved in the additive method. However, it is necessary that the resist pattern, which has a complicated cross-sectional shape, should be formed so that the resist and the metal film on the resist surface can be reliably exfoliated. This method involves such a problem that the number of production steps is consequently increased, and the production cost is consequently increased.
On the other hand, the mask film formation method is known, which uses a mask formed with openings corresponding to a wiring pattern without using the resist to form wirings. In the case of this method, the mask, which has the openings corresponding to the wiring portions, is allowed to make tight contact with a board surface. A metal film is formed on the entire surfaces of the mask and the board. After the film formation, the mask is detached to form the wirings. The accuracy of the formed wiring is determined by the accuracy of the mask. Therefore, it is possible to form the fine wirings in the same manner as in the lift-off method. When the sputtering method or the like is used, it is possible to form the metal film having the high heat tolerance. When the strength of the mask is high, it is possible to reuse the mask. Therefore, an advantage is obtained such that the production cost can be reduced.
However, the mask film formation method has involved the following problem. That is, for example, when a plurality of long linear wirings, which have a wiring width of about 2 μm and a length of about 2 mm and which are in a form of being aligned in parallel, are formed, there is given, for example, a state in which the tight contact of the mask with respect to the board is unstable, for example, a state in which any strain arises at portions of the parallel wirings, or the mask is in a floating situation without making the tight contact with the board. It is feared that any error may arise in the width of the wiring to be formed. It is feared that the wiring may be broken or the wiring may make contact with the adjacent wiring depending on the degree of the error.
Japanese Patent Application Laid-open No. 2001-254169 describes a mask having a wiring pattern composed of long linear wirings aligned in parallel. In the case of this mask, a fine rib, which spans a plurality of wiring patterns, is provided on the surface on the side opposite to the side on which the mask is stuck to the board, in order to reinforce the mask. In this way, it is intended that any strain is prevented from being generated at the portions of the parallel wirings.
However, when the rib, which spans the plurality of wiring patterns, is provided as in the mask described in Japanese Patent Application Laid-open No. 2001-254169, it is necessary that the film formation should be performed on the board surface while making the detour or bypass around the rib. Therefore, it is desirable that the distance between the board surface and the rib is long or separated. However, in order to increase the distance between the board surface and the rib, it is necessary that the mask body should be thickened. A problem arises such that it is difficult to form any fine wiring, because the film formation is hardly performed when the mask body is thickened.
In the case of the mask described in Japanese Patent Application Laid-open No. 2001-254169, the mask is formed by depositing a metal by means of the electroforming. Therefore, the mask body is made of the metal. When such a mask is used to form the metal wiring, then a problem arises such that it is difficult to reuse the mask, for the following reason. That is, it is feared that the mask body may be dissolved, because the metal film is exfoliated with a strong acid solution such as aqua regia or nitric acid when the metal film, which is formed on the mask surface, is exfoliated. It is feared that the opening portions of the mask for forming the wiring pattern may be buried by the metal film if the mask is reused without exfoliating the metal film formed on the mask surface.